1. Field of the Invention
The invention relates in general to the use of a flat conductive winding as an antenna and more particularly to a serpentine planar configuration for loop antenna having a high radio frequency cross section and in which the antenna terminals are closely adjacent to each other.
2. Description of the Prior Art
Loop antennas are of course one of the first designs employed for radiofrequency circuits. For example, D. L. Hings, "Omnipole Antenna," U.S. Pat. No. 3,325,805 shows in FIGS. 3 and 4 an inductance 29 enclosed with an electrostatic shield 30 having a base plate 31. Inductance 29 includes a first, second and third coil portions 32, 33 and 34, respectively connected in a series in a general U-shape. The entire inductance 29 has first and second ends 35 and 36 which are disposed closely adjacent to base 31 of electrostatic shields 30. The three coil portions 32, 33 and 34 each have an access lying in a plane 37. Shield 30 is rectangular and sides 38 and 39 parallel to plane 37.
FIGS. 5 and 6 show another embodiment wherein an inductance 46 is part of a transformer 47. Inductance 46 includes first, second and third coils 48, 49 and 50 connected in a series. Coils 48, 49 and 50 are disposed in a single plane with coils 48 and 50 disposed perpendicular to each other and with their ends closely adjacent.
Ware, "Radio Telephoning," U.S. Pat. No. 1,627,718 (1927) shows a receiving unit equipped with a comparatively small loop antenna of a conventional type depicted in FIGS. 1 and 2. Loop 5 as shown in FIG. 1 is double with each half of the loop wound in an opposite direction. Loop 5 may be connected as indicated in the circuit with a variable tuning condenser 76 and loosely coupled through coil 77 to the input circuit of detector 51. The receiver loop is shielded from local transmitter oscillations by any suitable means, but preferably by an electrostatic open circuited shielded cage 52 shown in FIG. 3.
Shield 52 is comprised of a special form of cage or coil with conductive material adapted to surround loop 5 and spaced apart from it. The preferred construction of the cage comprises two groups of spaced, parallel conductors connected in series with one end only of each group connected to a common ground connector 52'.
De Vail, "RF Transponder System With Parallel Resonant Interrogation Series Resonant Response," U.S. Pat. No. 5,608,417 (1997) shows in FIG. 1 antenna coils 4 and 6 formed on opposite surfaces of substrate 2. Each of coils 4 and 6 are serpentine coils formed on opposite sides of substrate 2 in generally rectangular spirals as you discuss as being the prior art. Inner ends 8 and 10 of coils of 4 and 6 are connected together by feedthrough 12, such a soldered or plated-through via or an insulation displacement connection that extends through an opening 14 in the substrate. Outer end 16 of coil 4 is connected to one terminal 18 of a transponder circuit which is implemented on IC chip 20, while the other end 22 of other coil 6 is connected to the opposite terminal 24 of transponder circuit 20 by another feedthrough 26 that extends through a corresponding opening in substrate 2.
Graue, "Loop Antenna," U.S. Pat. No. 1,615,755 (1927) shows in FIG. 1 outer and inner series of strips or bars 22 and 23 extending transversely between sides 14 of a cabinet. Strips or bars 23 in the inner series are in radial alignment with those in the outer series. The outer edges of bars 22 and the inner edges of bars 23 are notched at 24 and 25 as best shown in FIG. 2. The notches provide for retention of the successive convolutions of the coil so that the convolutions will not slip longitudinally on the supporting bars. The coil is indicated generally at 26 and is comprised of suitable conductor wound over outer strips 22 and under inner strips 25.
Libby, "Simulating Impedance System," U.S. Pat. No. 2,448,036 (1948) shows antenna 5 in FIG. 1 connected at one end 12 of an outer conductor of coaxial line coil 11. The other end 13 of antenna 5 is coupled to the outer conductor grounded to casing 9. The counterpoise 7 is connected to the outer conductor coaxial line 10 at end 14 with the opposite end 15 being grounded.
In radio frequency identification (RFID) tags, for example operating at frequencies of 125 kHz and 27.1 MHz, the transmission is predominantly through the magnetic field rather than through the electric field as occurs at 2.5 GHz. Therefore, magnetic inductively coupled coils are preferred rather than E-field transmitting antennae. The problem with inductive coils are that they are expensive to manufacture when fabricated in a single plane.
There have been two basic means of producing inductive RFID label in the past. The first is to use a wire coil with multiple turns. The wires are typically held with some sort of adhesive to give the coil rigidity. The coils are expensive and are difficult to handle and mass automated assembly is difficult.
The second method is to pattern a spiraling coil onto a substrate, such as copper onto a thin insulating substrate. This presents a problem in that the two ends of the coil are on opposite sides of the coil. The two ends must be brought into close proximity to each other in order to connect to the chip. This can be overcome by two methods. The first is to add a second conductor which can contact one end of the spiral and make a connection in close proximity to the other end. This too is expensive as the second conductor must be placed on the back of a substrate and feedthroughs are then required or an insulator must be placed over the first conductor so that the two conductors do not short. Both options are expensive to make on a mass scale.
Another way of getting around this problem is to have bonded wires cross the spiral without touching a coil. This also is difficult, costly and very limiting to the number of turns which can be included within the coil in a mass manufactured device.
What is needed then is a two dimensional configuration for a loop antenna in a single plane which can be manufactured all on one side or surface of an integrated surface substrate so that the antenna terminals may be closely positioned to each other.